Superoleophobic Surfaces Obtained via Hierarchical Metallic Meshes

Apr 14, 2016 - Institute of Technical Chemistry, Ural Division, Russian Academy of Science, ulitsa Academika Koroleva 3, Perm 614013, Russia. ABSTRACT...
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Superoleophobic Surfaces Obtained via Hierarchical Metallic Meshes Roman S. Grynyov, Edward Bormashenko, Gene Whyman, Yelena Bormashenko, Albina Musin, Roman Pogreb, Anton Starostin, Viktor Valtsifer, Vladimir Strelnikov, Alex Schechter, and Srikanth Kolagatla Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00248 • Publication Date (Web): 14 Apr 2016 Downloaded from http://pubs.acs.org on April 19, 2016

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Superoleophobic Surfaces Obtained via Hierarchical Metallic Meshes Roman Grynyova, Edward Bormashenko*a, Gene Whymana, Yelena Bormashenkoa, Albina Musina, Roman Pogreba, Anton Starostinb, Viktor Valtsiferb, Vladimir Strelnikovb, Alex Schechterc, Srikanth Kolagatlac a

Ariel University, Physics Department, Chemical Engineering and Biotechnology Department, P.O.B. 3, 407000, Ariel, Israel.

b

Institute of Technical Chemistry of Ural Division of Russian Academy of Science. c

Ariel University, Department of Chemical Sciences, 407000, Ariel, Israel.

E-Mail: [email protected] Abstract Hierarchical metallic surfaces demonstrating pronounced water and oil repellence are reported. The surfaces were manufactured with stainless-steel microporous meshes, which were etched with perfluorononanoic acid. As a result, a hierarchical relief was created, characterized by roughness at micro- and sub-micro scales. Pronounced superoleophobicity was registered with regard to canola, castor, sesame, flax, crude (petroleum) and engine oils. Relatively high sliding angles were recorded for 5 µl turpentine, olive and silicone oil droplets. The stability of the Cassie-like air trapping wetting state, established with water/ethanol solutions, is reported. The omniphobicity of the surfaces is due to the interplay of their hierarchical relief and surface fluorination. Keywords:

superhydrophobicity; superoleophobicity; omniphobicity; hierarchical

relief; metallic meshes; Cassie wetting. 1. INTRODUCTION Nature has created materials, objects, and processes that function from the macro-scale to the nano-scale.1 One of the most fascinating effects revealed by study of natural objects was the ability of lotus leaves to repel water and to keep their surface self-cleaned.2 The effects was named “superhydrophobicity”, i.e. the enhancement of hydrophobic properties due to roughness.3 Afterwards, a variety of biological objects, distinguished by the pronounced superhydrophobicity was discovered.2-6

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This discovery gave rise to the rapid development of synthetic surfaces demonstrating low adhesion and self-cleaning properties. Superhydrophobic surfaces are generally manufactured by developing complicated micro- and nano-scaled reliefs and controlling the surface chemistry of various materials.7 Wetting occurring on rough and chemically heterogeneous surfaces is described by the Cassie-Baxter and Wenzel models and their extensions developed for hierarchical surfaces.8-16 It was shown both experimentally and theoretically that the hierarchical multiscale topography of a relief strengthens the water repellency of surfaces, increases the apparent contact angle and decreases the contact angle hysteresis.15 In this case, the modified Cassie-Baxter equation considering the hierarchical topography of the relief should be applied.16 Rigorous thermodynamic grounding of the Cassie and Wenzel wetting models has been reported recently.16-21 It was demonstrated that the Cassie and Wenzel apparent contact angles arise from imposing the transversality conditions on the variational problem of wetting, and that they are independent on the volume of a droplet and external fields, such as gravity. The accurate implementation of the Cassie and Wenzel wetting models for describing the wetting of rough and chemically heterogeneous surfaces needs certain care; this problem was recently treated in detail in Ref. 21. Preparing reliefs demonstrating a highly stable Cassie wetting regime along with pronounced oleo-repellency (also called omniphobicity) remains a challenging experimental and technological task. Oils are usually characterized by low surface tensions, and by strong liquid-solid interactions; thus, they penetrate easily into elements constituting the relief and promote sticky Wenzel or Cassie-like impregnating wettings.17 Thus, omniphobic surfaces should demonstrate the highest possible stability of the Cassie wetting regime.22-27 A variety of advanced techniques have been reported, enabling the manufacturing of superhydrophobic surfaces.28-34 In spite of this, manufacturing superoleophobic metallic surfaces is very challenging, due to the innate hydrophilicity of metals.12-14,

35-46

Our paper reports a robust, two-stage technique giving rise to

omniphobic metallic surfaces, based on hierarchically roughened and fluorinated metallic meshes.

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2. EXPERIMENTAL SECTION Superoleophobic and superhydrophobic metallic surfaces were prepared according to the following protocol: stainless-steel 80400 meshes (see Figure 1), supplied by A. D. Sinun (Israel) were thoroughly cleaned with ethanol and acetone. The chemical composition of the stainless steel (AISI 304) is: Fe – 77 wt. %; Cr – 20 wt. %; Mn – 2 wt.%, other additives~1 wt.%. After cleaning, the meshes were immersed into a 20% water solution of hydrochloric acid (supplied by Alfa Aesar) for 18 hours. The total specific mass loss of meshes under the etching was 8.3

mg . cm2

The velocity of dissolution of the components constituting the stainless steel in the hydrochloric acid was variable; this resulted in the creating of the second (submicro) scale in addition to to the existing micro-scale of the mesh, giving rise to the twin-scaled hierarchical relief, shown in Figure 2. After the etching, the samples were dried at room temperature for 1 hour. To enhance hydrophobic and oleophobic properties of the samples, they were immersed in a 97% solution of perfluorononanoic acid (С9НF17О2, supplied by Alfa Aesar) for 4 hours. Then, the samples were dried at room temperature for 24 hours.

Figure 1. Scanning electron microscopy image of 80400 stainless-steel mesh before treatment. The scale bar is 50 µm.

Chemical composition of the reported fluorinated surfaces was studied with SEM/EDS (scanning electron microscope/energy dispersive spectrometer) carried out with SEM (model JSM-6510 LV). Raman spectra of meshes were obtained using

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Olympus BX41 setup with 532 nm laser and 1200 nm grating. A 50x Olympus objective was used.

A

B

Figure 2. Scanning electron microscopy images of rough stainless-steel meshes. A. The scale bar is 50 µm. B. The scale bar is 1 µm. 621.05 nm

120.29 nm

A

B

1.0µm

1.0µm

-716.00 nm

-71.38 nm

A

B

Figure 3. AFM 5 x 5 µm images of the mesh. A. The wires constituting stainlesssteel mesh before acid treatment. B. The same wires after the acid treatment. SEM imaging of the reported surfaces, presented in Figure 2, was accomplished by AFM study. Atomic force microscopy (AFM) imaging was performed by Nanonics MV2000 setup; a Si tip coated with Cr (which was of 20 nm in diameter) was used for establishing the surface morphology and roughness of the stainless-steel meshes before and after the hydrochloric acid treatment. Figure 3A shows an AFM scan of stainless-steel mesh wires before acid treatment. It is clearly seen, that the surface does not demonstrate roughness or

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etching features, a clear unevenness is observed on the surface which is of 120 nm in height. Figure 3B depicts the AFM scan of the wire after acid treatment; the surface apparently demonstrates etched features, with a number of trenches and unevenness recognizable. The observed trenches are 710 nm in depth on the average, and the overall height of the surface is 620 nm. Wetting properties of the surfaces were established with the following liquids: deionized water, silicone oil (Si(CH3)2O)n for melting and boiling points apparatuses, (supplied by Aldrich); nutritive oils: Canola, olive, flax and sesame; castor oil (supplied by Vitamed Israel), turpentine mineral spirit (supplied by Maoz, Israel) and crude oil (petroleum, supplied by Givot Olam Petroleum). Contact angles (apparent, receding and advancing) were measured by the RaméHart Advanced Goniometer Model 500-F1. Advancing and receding contact angles were measured by the tilted-plate method. Measurements were made on both sides of the droplet and were averaged. A series of 10 experiments was carried out for every aforementioned liquid. Sliding angles were established for 5 and 10 µl droplets with the tilted-plate method, defined as a minimal inclination angle of a surface at which a drop starts to slip downwards the plate. Surface tensions of olive, flax, sesame, and engine 20W/50 oils were measured with the pendant drop method. A syringe filled with oil was fixed vertically, and a liquid droplet of the necessary volume was suspended from the needle. When oil is suspended at the tip of a thin tube with the inner radius R, as shown in Figure 4, its shape is determined by the balance between the capillary and gravitational forces. As a result, the drop profile is described with the Young-Laplace equation:  1 1   = ρgz , +  R1 R2 

γ 

(1)

where ρ is the density of the oil, g is the gravity acceleration, R1 and R2 are the main radii of curvature of the droplet, and γ is the oil surface tension. Eq. 1, when rewritten in cylindrical coordinates (r, z), is represented as:

 1 r ′′ +  = ρgz , 2 1/ 2 2 3/ 2  r (1 + r ′ )   r (1 + r ′ ) 

γ 

(2)

where r = r(z), r′ = dr/dz, r′′ = d2r/dz2. Equation (5) could be solved numerically. The pendant drop is imaged and γ is considered as a fitting parameter.13,47 The surface

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tension γ is adjusted until the solution of Eq. (2) agrees with experimental results obtained with the droplet imaging. The pendant drop method is one of the most precise and commonly used methods for the measuring of the surface tension of liquids.

R

Needle

Droplet

r

z Figure 4. Pendant droplet method for measuring surface tension.

3. RESULTS AND DISCUSSION 3.1. Wetting properties of superhydrophobic and superoleophobic surfaces obtained with metallic meshes Superhydrophobic and superoleophobic metallic surfaces manufactured under sophisticated experimental techniques were reported recently by various groups.16, 4855

In the present paper, we implement an approach based on exploiting metallic

meshes as a substrate, which has an obvious advantage: meshes allow robust manufacturing of hierarchical surfaces. Indeed, the largest scale is already built in the surface; namely the scale of the mesh itself. This approach has been already successfully exploited by other groups for manufacturing superhydrophobic surfaces.44-46,52 Data related to apparent contact angles, contact angle hysteresis and sliding angles established for 5 and 10 µl droplets of various liquids, are summarized in

Table 1. It is recognized from this table that the reported hierarchical metallic surface

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demonstrates

the

distinct

Cassie

wetting,

accompanied

by

pronounced

superhydrophobicity and superoleophobicity for a diversity of oils including: canola, castor, flax, sesame, crude (petroleum) and engine oils (sliding angles as low as 3-5º established for 10 µl droplets of these oils are noteworthy); the surface demonstrates moderate oleophobicity against olive, silicone oils and turpentine (sliding angles ~812º for 10 µl droplets), as listed in Table 1. The contact angles registered for 5 µl droplets are naturally larger than those established for 10 µl ones; however, for canola, castor, crude and engine oils the angles are smaller than 10 degrees; thus, the reported surfaces are referred as superoleophobic. Sliding angles as high as 17-18º registered for 5 µl turpentine and silicone oil droplets are also noteworthy. It is plausible to suggest that the turpentine, olive and silicone oil/meshes systems demonstrates the so-called mixed wetting regime, combining the features of the Cassie and Wenzel wetting models, introduced and discussed in detail in Refs. 10, 11, 14, 19, 25.

Table 1. Wettability of reported metallic surfaces established for various liquids. Liquid

Surface tension, γ, mJ/m2

Apparent contact angle, ±1°

Advanc. contact angle, ±1°

Water* Canola oil** Castor oil*** Silicone oil**** Crude oil***** Turpentine****** Olive oil Flax oil Sesame oil Engine oil 20W/50

72.0 28-30 40.4 20 28-30 27 32.0±1 36.0±1 36.0±1 35.0±1

155 146 145 142 140 123 140 138 140 140

160 148 149 145 145 126 147 144 146 143

Receding Sliding Sliding angle, angle, contact 5 µl 10 µl angle, drop, drop, ±1° 126 115 125 117 86 99 122 117 124 102

±1°

±1°

5 4 8 18 6 17 12 10 11 6

2 3 3 12 4 8 10 4 5 4

*Surface tension is extracted from Ref. 56; **Surface tension is extracted from Ref. 57; *** Surface tension is extracted from Ref. 58; ****Surface tension is extracted from Ref. 59; *****Surface tension is extracted from Refs. 58, 60; ******Surface tension is extracted from Ref. 61. It should be stressed that the high apparent contact angle, such as depicted in

Figure 5, does not guarantee the oil-repellence of a surface. Low contact angle hysteresis and high stability of the Cassie wetting state are also necessary for

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providing easy sliding of droplets accompanied by self-cleaning properties of the surface.14,25

Figure 5. Castor-oil droplet with a volume of 8µL placed on the reported surface. High apparent contact angle is clearly seen.

3.2. Study of the Stability of the Cassie Wetting Regime with Water/Ethanol Solutions. It should be emphasized that it is more difficult to manufacture surfaces repelling liquids possessing low values of surface tension. It was suggested that there exists a certain critical value of the surface tension γc of a liquid at which the Cassie wetting is not already observed; and a droplet demonstrates the high-stick Wenzel wetting.62-64 For the experimental establishment of γc, several groups exploited water/ethanol solutions possessing controllable surface tension.62-64 Droplets of water/ethanol solutions were deposited gently on the studied surface, and the apparent contact angle was measured. The concentration of ethanol in the droplets was gradually increased and, as expected, the apparent contact angle decreased. At a certain concentration of ethanol, an abrupt change in the apparent contact angle was observed, indicating the onset of the Cassie-Wenzel wetting transition.62-64

Figure 6 displays the dependence of the apparent contact angle on the surface tension of the water/alcohol solutions, established at the surfaces depicted in Figures 2-3, for various concentrations of these solutions. The abrupt decrease of the apparent contact angle was registered at γ c ≈ 30 − 35

mJ , corresponding to the onset of the m2

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Cassie-Wenzel transition.14-25,62-64 Let us compare this result with the experimentally established liquids’ repellency of reported surfaces summarized in Table 1. It is recognized that the reported surfaces mostly repelled water and oils which possess surface tensions higher than γ c . The apparent contact angles established for water, castor, sesame, flax and engine oils were high and the sliding angles were low, evidencing the Cassie wetting regime. At the same time, high-stick wetting, resulting in high sliding angles, was observed for turpentine and silicone oil which demonstrate surface tensions much lower than γ c . On the other hand low sliding angles observed for the relatively low surface tension Canola oil are not fully consistent with the concept of the critical surface tension, corresponding to the onset of the Cassie-Wenzel transition. We conclude that use of the criterion of the critical surface tension for predicting the stability of the Cassie wetting (implying the oil-repellency of a surface) needs a certain measure of care. This is rather understandable, because the wetting properties of a surface depend not only on the surface tension of a liquid but on a triad of interfacial tensions, including the solid/liquid surface tension, which is regrettably not a well-established physical value.

180 160 140 θ*, degrees

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120 100 80 60 40 20 00

0

10

20

30

40

50

60

70

80

γ, mJ/m2 Figure 6. The dependence of the apparent contact angle θ * on the surface tension of water/ethanol solutions.

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Now consider the chemical composition of the reported surfaces, established with SEM/EDS spectroscopy and summarized in Table 2. It is recognized from the data supplied in Table 2 that the surfaces are strongly fluorinated.

Table 2. Surface composition of the surfaces (wt. ±0.5%), established with SEM/EDS spectroscopy. O 2.15

F 20.38

Al 0.25

Si 1.1

Cl 0.32

Cr 14.83

Mn 0.68

Fe 52.51

Ni 5.95

Mo 1.83

The fluorination of the reported surfaces is supported by the data extracted from the Raman spectra of virgin, hydrochloric and perfluorononanoic acids treated meshes, supplied in Figure 7. Consider first the Raman spectrum of the non-treated sample, shown in Figure 7A. Raman peak appearing at 616 cm-1 is attributed to FeO and Fe3O4 (see Ref. 66), whereas the peak observed at 652 cm-1 is reasonably attributed to α-Fe2O3 (see Ref. 67). The sharp peaks seen at 1056, 1292, 1432 and 1643 cm-1 are related to Cr2O3 lattice vibrations (it is noteworthy, that peaks inherent to α-Al2O3 lattice are very close to those of Cr2O3 thus it is not simple to make an accurate attribution of these peaks).68 However, the concentration of chromium in the meshes is much higher than that of aluminum, thus it seems plausible to relate these peaks to Cr2O3 lattice vibrations. Now consider the Raman spectra of the meshes treated by hydrochloric acid, depicted in Figure 7B. Very strong peak observed at 647 cm-1 is related to FeOOH group, inherent for FeO(OH)xCl1-x compounds arising under etching.69 This peak remains very strong after the treatment of meshes in perfluorononanoic acid (see

Figure 7C). The appearance of strong peaks at 1126, 1358 and 1539 cm-1 is usually related to the carbon contamination of the surface, arising from the atmosphere absorption of CO2.70 The Raman spectrum of the meshes treated by the perfluorononanoic acid is interpreted as flows: the peak registered at 1366 cm-1 may be reasonably attributed to antisymmetric CF3 vibration or stretching CF2 vibration71; whereas very strong peak at 1581 cm-1 is inherent to COO- group inherent for carboxylic acid salts (see Figure

9C).72 The very strong peak seen at 907 cm-1 is attributed as the combination band

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inherent for CF2 groups.73 These findings indicate the reaction of the organic acid with the freshly prepared reactive areas arising from the previous passivation dissolution of meshes by hydrochloric acid solution. Thus, it is reasonable to relate the observed omniphobicity, which is much more pronounced than that reported in our recent work,51 to both the hierarchical relief depicted in Figures 2-3 and the surface fluorination.41, 43, 51-52, 65

616 652

Figure 7. Raman spectra of the virgin (non-treated) (A), hydrochloric acid (B) and perfluorononanoic acid (C) treated meshes It should be emphasized that the superoleophobic properties were kept by meshes for 60 days, when stored under ambient conditions (the average humidity was 40% RH); this observation evidences the temporal stability of wetting properties of meshes, important in the context of industrial applications.

Conclusions We present omniphobic stainless steel surfaces. The surfaces manufactured with micro-porous meshes repelled a broad diversity of organic liquids including canola, castor, flax, sesame, crude (petroleum) and engine oils, and water. Thus, the reported surfaces may be considered as omniphobic, i.e. surfaces repelling low

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surface tension liquids. At the same time, weak oleophobicity, accompanied by relatively high sliding angles, was observed for turpentine, olive and silicone oils. The stability of the Cassie-like wetting was investigated for the reported surfaces with the use of water/ethanol solutions, allowing the gradual control of the surface tension of a liquid. The critical surface tension of a liquid γ c corresponding to the onset of Cassie-Wenzel transitions was established experimentally. We conclude that the criterion of the critical surface tension of a liquid corresponding to the breaking of the Cassie wetting state generally works; however exceptions are possible. Turpentine and silicone oil droplets possessing surface tensions lower than

γ c demonstrated the so-called mixed wetting regime, combining the features of both the Cassie and Wenzel wetting models. On the other hand, low sliding angles observed for the relatively low surface tension Canola oil were not completely consistent with the concept of the critical surface tension corresponding to the onset of the Cassie-Wenzel transition. The observed superoleophobicity was achieved by the two-step process, under etching of meshes with perfluorononanoic acid, giving rise to the hierarchical microand sub-micro-scaled relief. The Raman spectra and SEM/EDS spectroscopy indicated noticeable fluorination of the surface. It seems reasonable to attribute the observed omniphobicity of the surfaces to the combination of the well-developed hierarchical relief and the pronounced surface fluorination.

Acknowledgements The authors are thankful to Mrs. N. Litvak for SEM imaging and SEM/EDS spectroscopy. The work was financially supported by the Foundation for Basic Research (grant No. 14-03-96009), the Foundation for Assistance to Small Innovative Enterprises in the scientific and technical field and of Ministry of Education of Perm Region (Agreement № С-26/004.06).

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Graphical Abstract

Omniphobic surfaces obtained with a robust two-stage process are reported.

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